The present invention is related to field emission devices, particularly to gated field emitters, and more particularly to the formation of a high aspect ratio nanofilament field emission device with and without gate passivation, and, where needed, plating enhancement.
In recent years substantial effort has been directed to the use of gated field emission devices in flat panel displays and in vacuum microelectronics for radiation hard performance, lower power electron sources for ion cells, and in ultrasensitive chemical sensors. For example, it is forecast that flat panel displays will be a 10-20 billion dollar per year market by the turn of the century. Currently, the flat panel displays primarily involve active matrix liquid crystals, and field emission displays are one of the leading contenders. In field emission cathodes, both the materials of the emitter and the gate, and the geometry of emitter-gate structure are very important. By forming an extremely sharp (needle-like) emitter tip centered in a small grid (gate) hole or via, emission is more uniform among the emitters and the turn-on voltage, at which electron emission is initiated, is lowered.
Recent efforts in the development of gated field emitters having sharp tips have been directed to forming the emitter by electroplating, the grid or gate metal being used in some techniques as a counter electrode. Such recent efforts which involved the formation of "nanocones" and "nanofilament" type emitters are exemplified by copending U.S. application Ser. No. 08/847,087, filed May 1, 1997, now U.S. Pat. No. 5,891,321, entitled "Electrochemical Sharpening of Field Emission Tips", and copending U.S. application Ser. No. 08/847,085, filed May 1, 1997, pending entitled "Use of a Hard Mask for Formation of Gate and Dielectric Via Nanofilament Field Emission Devices," each assigned to the same assignee. Also, selective etching of nuclear tracked materials to create a mold for electroplating nanofilament structures having a diameter of 0.5 to 1.0 micron heights has been demonstrated. See U.S. Pat. No. 5,462,467 issued Oct. 31, 1995 to J. M. Macauley et al.
These recent efforts have established that electrochemical deposition can be utilized to form nanofilament structures in oxide vias having diameters of 50-200 nm, and aspect ratios (the ratio of the height of the filament to its diamenter) of 5-10 results in a cylindrical shaped nanofilament which is self-aligned to a gate electrode on top of the dielectric. However, in order to make these prior processes more manufacturable, it has been found that it is necessary to passivate the gate structure, thereby allowing overplating to occur, thus compensating for nonuniformity in deposition rate. Furthermore, since the electroplating of the emitter may be done on a high resistivity film or resistor layer, adhesion and initiation of the electroplated material is an issue, depending on the composition of the layer on which the emitter is to be deposited. Also, the tip of the electroplated structure must form a sharp point, centered in the gate metal via, substantially in the plane of the gate metal layer.
The present invention enables manufacturably of gated emitters by a process for creating a nanofilament field emission device which involves gate metal passivation and plating enhancement. The formation of high aspect ratio, electroplated nanofilament structure devices for field emission flat panel displays, for example, requires the formation of a via in a dielectric layer which is self-aligned to a gate metal via structure on top of the dielectric layer. The desired diameter of the via in the dielectric is on the order of 50-200 nm, with an aspect ratio defined by depth divided by via diameter of 5-10. Once the via in the dielectric is created, the gate metal is passivated, after which a plating enhancement layer may be deposited in the bottom of the via. The nanofilament is then electroplated in the via, after which the gate passivation layer is removed, the dielectric etched back, and the nanofilament sharpened. The process of this invention provides more tolerance in the electroplating and sharpening than the processes of the above-referenced copending applications.